The present invention relates to integrated circuits, and, more particularly, to a CMOS temperature sensor adapted for operation over a wide range of supply voltages. For example, the temperature sensor according to the present invention can operate within a range of 1.8 V to 7 V at a current consumption of about 2.8 mA.
Temperature sensors are incorporated in a number of electronic circuits for a variety of purposes. For example, dissipation by integrated circuits exceeding a safe temperature limit needs to be signaled so that the dissipation can be controlled. In addition, the operating temperature range for a particular electronic system may have to be limited.
More particularly, temperature detectors have been used as security arrangements for monitoring the operating temperature of non-volatile EEPROM memories used in Smartcards. Smartcards are plastics cards having an integrated circuit, such as a memory circuit or a microcontroller, embedded therein. These cards are increasing in popularity for a variety of applications, including credit/debit cards, POS cards, telephone cards, access control cards, etc.
Smartcards are to meet strict security requirements, and this has resulted in a constant development of intrusion-defeating arrangements for their improved effectiveness. One possible form of intrusion is to expose the card to a heat source to alter the operation of the integrated circuit contained therein. Many intrusions into smart cards can be attempted by a technique known as differential power analysis (DPA), which is readily understood by one skilled in the art.
Also known is the use of hardware or firmware security arrangements. These arrangements are being regularly designed for improved security. Such arrangements include using advanced 0.35-micron technologies to minimize both the size and the consumption rate of the integrated circuit, as well as relative variations in its operational parameters. When combined with a screening metallization level, this makes DPA attempts less likely to succeed.
Other such security arrangements include using specific operational software for the internal timing of the integrated circuit, whereby high variability is introduced in the operation scans when such operations are carried out. A modular design may also be used which enables the circuit hardware to be quickly modified for immediately counteracting new forms of intrusive attempts.
Yet another security arrangement includes using a series of hardware mechanisms, such as thermal protectors, for example, which enable the resident integrated circuit applications to detect and react as appropriate to the occurrence of any operational conditions which may look like an intrusive attempt or is actually a factual indication of an intrusive attempt.
There is a need to improve resident thermal protectors for an integrated circuit requiring such protection. In general, temperature can be measured in integrated circuits on the basis of a voltage differential DVbe which is proportional to temperature (PTAT), and a voltage value which is proportional to Vbe and decreases linearly with temperature (CTAT). In other instances, a comparison of a temperature-stable reference voltage which may be obtained from a bandgap generator or by compensation between a voltage VPTAT and a voltage VCTAT, and a voltage that varies linearly with temperature, is performed.
There also exist circuits effecting a direct comparison of a voltage which increases linearly with temperature and a voltage which decreases linearly with temperature. Such is the case, for example, with the circuit shown schematically in FIG. 1, which illustrates a temperature sensor formed with bipolar technology. This type of sensor is described in IEEE Journal of Solid-State Circuits, Vol. 31, No. 12, December 1996, page 1912, for example.
The sensor of FIG. 1 illustrates how a voltage VPTAT and a voltage VCTAT can be compared to cause the output to switch over at a desired temperature. In all cases, an analog signal is generated which will cross zero at the desired temperature.
A further prior art embodiment implementing a temperature sensor is shown schematically in FIG. 2. This embodiment shows that a voltage VPTAT and voltage VCTAT can be used to generate two currents. A first current is temperature stable, and a second current is linearly dependent on the temperature. The two currents are compared together to determine whether a desired temperature has been reached. The circuit shown in FIG. 2 is implemented with CMOS technology, and is described in an article by Szekely et al., IEEE Transactions on VLSI Systems, Vol. 5, No. 3, September 1997, page 270, for example.
The circuit illustrated in FIG. 2, although achieving its objective, has a drawback in that generated currents are dependent on the resistance of a resistor R1, which is usually a few megaohms. However, this value can have a substantial process spread.
In addition, the thermal coefficient of the resistor R1 makes generating currents with a desired dependence on temperature difficult to achieve. To overcome such problems, polysilicon resistors are used which exhibit negligible process and thermal coefficient variations. However, in view of the low resistivity per circuit area square (approximately 1-6 Ohms/square) of such resistors, very large silicon areas or relatively large currents must be used, resulting in increased cost.
In view of the foregoing background, it is therefore an object of the present invention to provide a temperature sensor with CMOS technology that has adequate structural and functional features to allow a desired temperature to be detected rapidly and accurately, while reducing the area requirements for the sensor.
This and other objects, features and advantages in accordance with the present invention are provided using CMOS elements in place of a resistor commonly used by conventional sensors for generating the comparison currents.
The CMOS temperature sensor includes a first circuit portion for generating an increasing voltage as the temperature to be sensed increases, and a second circuit portion for generating a decreasing voltage as the temperature to be sensed increases. A comparator compares such voltage values together and outputs an electric signal based upon a predetermined temperature being reached.